There are too many variables that are difficult for the fabricator to
control that can affect impedance measurements such that you can rely on
extensive mathematical calculations alone to gaurantee the impedance
range. Smoothness of copper etching, copper thickness variation, plating
thickness variation and type (nickel thickness variation is much more
lossy than IAg for example), dielectric properties of substrate, etc,
etc.
You need to test the representative coupon to ensure the desired
impedance characteristics are met. Any of these physical parameters can
fall within a calculated range, but a stackup can result in impedance
measurements out of range for functionality of the circuit.
It is usually possible to test the impedance of a particular circuit(s)
during bare board fabrication testing to ensure that 100% of the lot
meet the desired impedance characteristics.
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Haynes, Kim
Sent: Monday, December 03, 2007 1:37 PM
To: [log in to unmask]
Subject: Re: [TN] Calculated Impedance vs. CI with coupon
NO!
All processes move within a window and all process need to be tracked.
The coupon measurements are reality. Math is a wonderful tool. But if
the shop's real output does not fall within the requirement window,
improving the calculations will not improve the product.
Regards,
Kim Haynes
Interface and Clock Products
High-Speed Serial Link Applications
Texas Instruments, Inc.
214-567-2057 Telephone
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Avigdor JAkubinsky
Sent: Monday, December 03, 2007 1:09 PM
To: [log in to unmask]
Subject: [TN] Calculated Impedance vs. CI with coupon
Hello friends,
It is always the question of controlling (CI) with Coupon measurement on
the stack.
Would any of you The Experts trust a meticulous Calculated approach
without actually measuring the outcome impedance Out of the coupon?
I would appreciate your pointer/Link to any written facts publishes at
any given year along with your personal opinion.
Best Regards
Avigdor Jakubisnky
Engineering Mgr.
www.fibrolan.com
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